Portable electronic devices are typically battery powered, and will cease operating properly when battery voltage drops below a certain voltage level. For many such devices, the only consequence of low battery voltage is that the device can unexpectedly cease operation. In more sophisticated devices, however, more serious consequences can occur. For example, in computing devices, such as tablet devices and mobile phone devices that have computing environments, data can be lost or corrupted if a voltage drop-out event occurs where voltage drops below a level necessary for the circuits in the device to properly function. In some devices that transmit radio signals, unreliable operation of radio circuitry can cause a radio transmission to fall out of a regulated set of parameters and possibly affect signals on nearby frequencies. Some devices can experience significant changes in voltage due to changing electric current demands as components of the device turn on and off. High current demand by a device causes the battery voltage to drop to a point that can damage the cell or cells of the battery, depending on the particular chemistry used in the battery cells. The voltage drop can be modeled as an internal impedance of the battery, and it can change based on a variety of conditions, including battery state of charge, age of the battery, temperature of the battery, and so on.
To avoid the negative effects of voltage drop, it is common to design devices to shut off when the voltage drops below a threshold voltage. This shut off voltage threshold can be selected based on likely conditions, and may include some overhead to account for worse battery conditions. As a result, there is a trade-off; the device is protected from undesired operation at the potential expense of leaving charge capacity in the battery. In addition, it is possible for battery impedance to decrease during operation, such as when a very cold battery is used. Temperature affects battery impedance inversely; as the temperature drops, generally the battery impedance increases. Accordingly, when a cold battery begins to warm, such as due to self-warming from use, its impedance tends to decrease. Thus, setting a fixed shutdown voltage level that is high enough to account for cold batteries likewise can result in not utilizing all the available charge capacity.
Accordingly, there is a need for a method and apparatus that can use more battery charge capacity over the conventional solutions while avoiding shutdown of the device powered by the battery.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.